Natural gypsum is also known as calcium sulfate dihydrate, terra alba or landplaster. Calcined gypsum is obtained by removing part of the water associated with the gypsum crystal. Synonymous terms for calcined gypsum are Plaster of Paris, stucco, calcium sulfate half-hydrate and calcium sulfate hemihydrate. Calcined gypsum, stucco and hemihydrate are the most commonly used terms, and they are used interchangeably in this application. When gypsum is mined, the natural rock is found in the dihydrate form, having about two water molecules associated with each molecule of calcium sulfate. In order to produce the stucco form, the gypsum can be calcined to drive off some of the water of hydration represented by the following equation:CaSO4.2H2O→CaSO4.½H2O+ 3/2H2O
Calcium sulfate hemihydrate is obtained by calcination to remove the associated water molecules. The hemihydrate is produced in at least two crystal forms. Alpha-calcined gypsum is made by a slurry process or a lump rock process whereby the calcium sulfate dihydrate is calcined under pressure. The alpha-calcined gypsum forms less acicular crystals than beta-calcined gypsum, allowing the crystals to pack tightly together, making a denser and stronger plaster. The crystal morphology of alpha hemihydrate allows water to flow easily between the crystals, requiring less water to form a flowable slurry. More elongated irregular shaped crystals are characteristic of the beta-hemihydrate, which is obtained by calcining gypsum at atmospheric pressure. This crystal structure results in a less dense product because the crystals are more loosely packed. The beta form also requires more water to fluidize the calcined gypsum. If the calcining of the dihydrate is performed at ambient pressure, the beta form is obtained and the cost is relatively low compared to the alpha-calcined gypsum.
Synthetic gypsum, which is a byproduct of flue gas desulfurization processes from power plants, is also useful. Flue gas that includes sulfur dioxide is wet scrubbed with lime or limestone. Calcium from the lime combines with the sulfur dioxide to form calcium sulfite.CaCO3+SO2→CaSO3+CO2 Via forced oxidation, the calcium sulfite is converted to calcium sulfate.CaSO3+2H2O+½O2→CaSO4.2H2O
Synthetic gypsum is converted into calcium hemihydrate by calcination. While there are advantages to using synthetic calcined gypsum because this significantly decreases the environmental footprint, adding synthetic calcined gypsum usually increases water demand and more water is needed to prepare a workable gypsum slurry. Increasing the water demand is not desirable because this may affect unfavorably the strength of a gypsum product and may also increase the time and energy needed for evaporating the excess of water from the gypsum product.
Diethylene-triamine-pentaacetic acid (DTPA) is also known as a set retarder, but the dry powdered forms have been found to be ineffective. For example, U.S. Pat. No. 4,661,161 to Jacacki (“Jacacki”) teaches the addition of a liquid form of diethylenetriamine pentaacetic acid (“DTPA”). U.S. Pat. No. 8,343,273 to Lettkeman et al., the entire disclose of which is incorporated herein by reference in its entirety, provides methods for utilizing DTPA as set retarder in compositions with beta stucco from calcined natural rock gypsum.
U.S. patent application Ser. No. 14/514,961 to Dennis Lettkeman et al. incorporated herein by reference in its entirety discloses modified synthetic calcined gypsum which has been coated with DTPA and methods for spray-coating with DTPA.
Oil is obtained in field from oil wells. Setting up an oil well includes oil well cementing which is a process of mixing a water-cement slurry and pumping it down through steel casing to critical points located in the area around the well casing, in the open well or below in the fractured formations.
Cement, such as for example Portland cement, is conventionally used in oil well cementing products. However, cement shrinks upon setting which affects negatively the tightness and strength of a cement plug in the oil well. This may lead to leakage and other problems. Mixing alpha calcium sulfate hemi-hydrate with cement improves the oil well cement composition because the composition no longer shrinks and may expand instead. This ensures that the cement plug is tight and minimizes the risk of leakage.
However, there remains the need for oil well cementing products that would expand upon setting and create a tight plug. There is also a need for oil well cementing products that would have sufficient fluidity to be pumped into an oil well, yet at the same time they must have a short setting time so that the cementing product sets soon after being pumped into the oil well.